With the development of the power industry, the capacity of thermal power plants is getting larger and larger, and new medium and high alloy steels are emerging, such as martensitic heat-resistant steel SA335-P91 and pearlitic heat-resistant steel SA335-P22.

Comparison of chemical composition and mechanical properties of SA335-P91 and SA335-P22 steels:

Chemical composition(%)

Grade	C	Si	Mn	Cr	Mo
SA335-P91	0.08～0.13	0.05～0.20	0.30～0.60	8.00～9.50	0.85
SA335-P22	≤0.15	≤0.50	0.30～0.60	1.90～2.60	0.78～1.13

Mechanical properties

Grade	Yield Strength(min) (Mpa)	Tensile Strength(min) (Mpa)	Elongation(min)(%)	Hardness(max)(HB)
SA335-P91	415	585	20	250
SA335-P22	205	415	30	163

SA335-P22 is a pearlitic heat-resistant steel with high heat strength, thermal stability and corrosion resistance, and the martensite transformation temperature is 430°C-450°C. SA335-P91 is a martensitic heat-resistant steel whose structure is at the boundary of M-F, which has higher heat strength, thermal stability and corrosion resistance than the former.

SA335-P91+SA335-P22 dissimilar steel welding problems
1. The higher the alloy content, the easier it is to form hardened structure.
The Cr, Mo and V in the steel make the C curve strongly shift to the right, increasing the hardenability of the steel. During the post-weld cooling process, the martensite brittle structure is generated in the weld zone and the heat affected zone, and the high alloy side The cold cracking tendency of the weld and the near seam area is greater.

2. Carbon migration under long-term high temperature
In the joints operating in the heat-affected zone and high temperature, due to the large difference in chemical composition of the fusion zone, both sides of the fusion zone are prone to carbonation and decarburization. The high alloy side is heavily carbonized to produce coarse carbides, and the low alloy side is severely desorbed. The carbon forms a wide, low strength F-belt that forms a low strength brittle joint.

3. Heat affected zone softening
During the welding process, the base metal is heated to a tempering zone near Acl where austenite (A) decomposition products, polymerized carbides and a large amount of ferrite from martensite to pearlite appear extremely close. The annealed state of steel is called the softening zone. Under the long-term high temperature load, the permanent strength and plasticity of this area are greatly reduced, and the depth of the softened layer is proportional to the time of staying near Acl.

Methods of solution

Prevent cold cracks
Pre-weld preheating and control of interlayer temperature and reduction of line energy can reduce and reduce welding thermal stress and post-weld residual stress; keep the workpiece temperature higher than martensite transformation temperature during welding, and avoid austenite to horse The transformation of the body to prevent the formation of hardened structures; the selection of low-hydrogen electrodes and reasonable welding processes.

Overcome, reduce carbon migration and reduce softening depth
The electrode with chemical composition between the two or the transition zone of chemical composition in the weld metal is selected to control the heat treatment process parameters to prevent the carbon migration from increasing and the softening band widening due to excessive temperature.